CH. XXXVIII.] THE KIDNEY ONCOMETER 571 



are tubular and lined by columnar epithelium; their secretion 

 dilutes the semen. Very little is known of the function of the 

 prostate; it often enlarges and becomes calcareous in old age, and 

 gives rise to discomfort and difficulty in micturition. Its removal 

 under these circumstances is a most beneficial operation. 



The Nerves of the Kidney. 



These are derived from the renal plexus of each side. The renal 

 plexus consists of both medullated and non-medullated nerve-fibres, 

 with collections of ganglion cells. Fibres from the anterior roots of 

 the eleventh, twelfth, and thirteenth dorsal nerves (in the dog) pass 

 into this plexus. They are both vaso-constrictor and vaso-dilator in 

 function. The nerve- cells on the course of the constrictor fibres are 

 situated in the coeliac, mesenteric, and renal ganglia; the nerve-cells 

 on the course of the dilator fibres are placed in the solar plexus and 

 renal ganglia. We have, at present, no knowledge of true secretory 

 nerves to the kidney, and the amount of urine is influenced, to a 

 certain extent at any rate, by the blood-pressure in its capillaries. 

 We shall, a few pages hence, however, see that the amount of urine 

 does not depend wholly on the height of the blood-pressure ; and one 

 very striking fact in this relation may be mentioned now, namely, 

 that if the blood-pressure is increased without allowing the blood to 

 flow, the amount of urine formed is not increased ; this can be done 

 by ligaturing the renal vein ; the blood-pressure within the kidney 

 then rises enormously, but the flow of urine stops. 



The Kidney Oncometer. 



This is an instrument constructed on plethysmographic principles, 

 by means of which the volume of the kidney is registered. The 

 general characters of this instrument are described in the diagrams 

 on p. 310. The special form introduced by Eoy for the kidney is 

 shown in fig. 376. Eoy's instrument, however, is but seldom used at 

 the present day. An air oncometer, connected with a Marey's 

 tambour or a bellows recorder (like that figured for the spleen on 

 p. 311), is much less complicated, and gives better results. 



It is found that the effect on the volume of the organ of dividing 

 or stimulating nerves corresponds to blood-pressure. If a rise of 

 pressure in the renal artery is produced by constriction of the renal 

 arterioles, this is accompanied by a fall of pressure in the renal 

 capillaries and a shrinkage of the kidney. Increase in the volume 

 of the kidney is produced by the opposite circumstances. 



The accompanying tracing (fig. 377) shows that in a normal 

 oncometric curve from the kidney there is a rise of volume, due to 



